This is part of a series of columns by retirement expert Dan Solin. Please bring him your questions in the comments box and he will answer as many as he can.

Is this a good time to invest, or should you sit on the sidelines until the market has "bottomed out"? This is the most common question I am asked.

It would be great if there was a way to tell when the market had reached its low. If you could do this, you would be able to buy stocks when the markets were taking off and retreat to risk-free investments, like cash and Treasury bills, in down markets.

Unfortunately, the data on timing the markets is very dismal.

One large study looked at more than 15,000 predictions by 237 market timing newsletters over a 12-year period. At the end of the period studied, 94.5% of the newsletters went bust. Not very impressive.

The financial media likes to hype stories suggesting that the markets are tanking or are poised for a rebound. These predictions are usually inaccurate and generally unreliable.

Here's a better question for you to consider: Should you be in the markets at all?

If you will need 20% or more of your money within 5 years, you should confine your investments to FDIC insured certificates of deposit or Treasury bills.

Your issue is not when you should be exposed to market risk. You should be out of the markets entirely until you can leave your money invested for at least that period of time.

If you have a longer time horizon, you should always be invested in the markets. However, it is critically important that you are only exposed to market risk that you can afford to assume.

Go to www.smartestinvestmentbook.com. Click on "Asset Allocation." Take the free asset allocation questionnaire. The answer to the questionnaire will determine how much of your money should be invested in stocks and how much should be invested in bonds.

Once that determination has been made, invest in a globally diversified portfolio of low-cost index funds or exchange traded funds for both the stock and bond portion of your portfolio.

You should either be out of the markets altogether, or invested in an asset allocation appropriate for you. There is no middle ground.

Dan Solin is the author of The Smartest Investment Book You'll Ever Read and the New York Times bestseller, The Smartest 401(k) Book You'll Ever Read. Visit his website at Smartestinvestmentbook.com.